Method for preparing composite flexible graphite material

ABSTRACT

An improved method enables preparation of articles for a variety of applications, including embossing or otherwise shaping to form a variety of articles. A plurality of thin, preferably resin-impregnated, flexible graphite sheets are arranged to form zones in a composite material having a graded density or other characteristic. Variation of properties between zones in the stock material formed is useful for achieving a desired set of properties in the formation of articles such as those useful in electrochemical fuel cells and double-layer capacitors.

TECHNICAL FIELD

[0001] The invention relates to a material which can be used inpreparing flexible graphite articles of controlled properties, such asdensity, to facilitate a further processing step. In a preferred form,the material will provide a stock material useful for embossing into aprecise surface pattern or structure, while providing close tolerancecontrol of important product characteristics. More particularly, theinvention relates to methods and materials enabling the preparation ofshaped elements having predetermined density gradation and/or detailnecessary for their intended functions. The invention enables theachievement and control of density and/or structural definition suchthat performance per unit weight can be improved in an array of finalproducts, at least some of which are useful as components inelectrochemical supercapcitors and fuel cells.

BACKGROUND OF THE INVENTION

[0002] Electrochemical devices like supercapacitors and fuel cells areforeseen by some as necessary to the commercial realization oflow-emission vehicles as well as a number of stationary power needs.Supercapacitors are useful for storing and releasing large bursts ofenergy, while fuel cells cleanly and efficiently convert suitable fuelsto electrical energy. The unique advantages of each type of device makethem, alone and together, promising for many power applications. In allcases, a balance must be struck between weight and performance, and itwould be desirable to adjust manufacturing procedures of currentconstruction materials to assure that both concerns are effectivelyaddressed to provide a net improvement in the operation and/or economyof these devices.

[0003] The ability to design and produce key elements with consistentsurface topography, density, electrical, thermal and strengthcharacteristics depends on the provision of procedures and materialsthat enable close tolerance control. Tolerance control is of criticalimportance because tolerance variations can accumulate. It is desirableto establish procedures for treating materials that enable achievingtarget properties. Where this cannot be done, it is frequently necessaryto compensate for out of tolerance regions during the designphase—leading to a less-than-ideal compromise on key structural andperformance criteria. Often when a choice is made to add additionalmaterial in order to assure that minimum structural specifications aremet, adds to the cost of the article being produced and decreases itseffectiveness both per unit cost and per unit weight. In each instancewhere material distribution is altered to achieve improved thermal orelectrical characteristics, other characteristics are affected andcompromised. It would be desirable to enable more nearly meetingcompeting design criteria with close tolerance control.

[0004] Double-layer capacitors, sometimes also called ultracapacitorsand supercapacitors, are capable of rapidly charging to storesignificant amounts of energy and then delivering the stored energy inbursts on demand. To be useful, they must, among other properties, havelow internal resistance, store large amounts of charge and be physicallystrong per unit weight. There are, therefore, a large number of designparameters that must be considered in their construction. It would bedesirable to enable procedures for producing starting materials forcomponent parts that would address these concerns such that the finalsupercapacitor assembly could be more effective on a weight and/or costbasis.

[0005] Capacitors of the double-layer type generally include two porouselectrodes, kept from electrical contact by a porous separator. Both theseparator and the electrodes are immersed within an electrolytesolution. The electrolyte is free to flow through the separator, whichis designed to prevent electrical contact between the electrodes andshorting of the cell. Current collecting plates are in contact with thebacks of active electrodes. Electrostatic energy is stored in polarizedliquid layers, which form when a potential is applied across theelectrodes. A double layer of positive and negative charges is formed atthe electrode-electrolyte interface.

[0006] The use of graphite electrodes in electrochemical capacitors withhigh power and energy density provides a number of advantages, buteconomics and operating efficiency are in need of improvement.Fabrication of double layer capacitors with carbon electrodes is known.See, for example, U.S. Pat. No. 6,094,788, to Farahmandhi, et al., U.S.Pat. No. 5,859,761, to Aoki, et al., U.S. Pat. No. 2,800,616, to Becker,and U.S. Pat. No. 3,648,126, to Boos, et al. The art has been utilizinggraphite electrodes—but not flexible graphite sheets—for capacitors ofthis type for some time and is still facing challenges in terms ofmaterial selection and processing.

[0007] A continuing problem in many carbon electrode capacitors,including double-layer capacitors, is that the performance of thecapacitor is limited because of the internal resistance of the carbonelectrodes. While the use of carbon in the form of flexible graphitesheet has several advantages, it is desired to further reduce cellinternal resistance. Internal resistance is influenced by severalfactors, including the high contact resistance of the internalcarbon-carbon contacts, the contact resistance of the electrodes with acurrent collector, the surface and internal pore structure of the carbonand the material thickness. Because high resistance translates to largeenergy losses in the capacitor during charging and discharge, and theselosses further adversely affect the characteristic RC(resistance×capacitance) time constant of the capacitor and interferewith its ability to be efficiently charged and/or discharged in a shortperiod of time, it would be desirable to provide construction materialsand methods that would facilitate reductions in the internal resistance.

[0008] Material selection and processing problems are also prevalent inthe field of fuel cells, where construction of flow field plates (FFP's)and gas diffusion layers (GDL's) have been made of flexible graphitefoil has been suggested due to an overall favorable combination ofphysical and electrical properties. Among the fuel cells utilizing thistype of construction are ion exchange membrane fuel cells. Of these,proton exchange membrane (PEM) fuel cells are of particular interest.Cells of this type produce electricity through the chemical reaction ofhydrogen with oxygen from the air. Within the fuel cell, electrodesdenoted as anode and cathode, surround an ionically-conducting polymer(performing the function of an electrolyte) to form what is generallyreferred to as a membrane electrode assembly (or MEA). In some cells,the electrode component will also function as a GDL. A catalyst materialstimulates hydrogen molecules to split into hydrogen atoms and then, atthe membrane, the atoms each split into a proton and an electron. Theelectrons are utilized as electrical energy. The protons migrate throughthe electrolyte and combine with oxygen and electrons to form water.

[0009] A PEM fuel cell is advantageously formed of a membrane electrodeassembly sandwiched between two graphite flow field plates.Conventionally, the membrane electrode assembly consists ofrandom-oriented carbon fiber paper electrodes (anode and cathode) with athin layer of a catalyst material, particularly platinum or a platinumgroup metal coated on isotropic carbon particles, such as lamp black,bonded to either side of a proton exchange membrane disposed between theelectrodes.

[0010] In operation of one of these PEM cells, hydrogen flows throughchannels in one of the flow field plates to the anode, where thecatalyst promotes its separation into hydrogen atoms and thereafter intoprotons that pass through the membrane and electrons that flow throughan external load. Air flows through the channels in the other flow fieldplate to the cathode, where the oxygen in the air is separated intooxygen atoms, which join with the protons migrating through the protonexchange membrane and the electrons through the circuit. The result isthe generation of current and the formation of water. Since the membraneis an electrical insulator, the electrons cannot directly cross themembrane, but seek the least resistance and travel through an externalcircuit which utilizes the electricity before the electrons join theprotons at the cathode. An air stream on the cathode side is onemechanism by which the water formed by combination of the hydrogen andoxygen can be removed. Combinations of such fuel cells are used in afuel cell stack to provide the desired voltage.

[0011] One factor limiting the use of flexible graphite materials ascomponents for PEM fuel cells is the definition of a pattern embossed onthe flow field plates, which, if not desirably precise and regular, cancreate anomalies in fuel cell operation, by either permitting leaking offluids, or not permitting sufficient fluid flow through the fuel cell.Aggravating this problem are several opposing problems. Among these, arethe needs for overall structural integrity and for the surface opposedto the embossed surface to be relatively dense to reduce permeability.Thus, there is a need for a suitable structural material, which canreadily be shaped at one surface to conform to the surface of anintricately-shaped mold and yet have another surface that issufficiently dense as to be impermeable under the conditions ofoperation to yield an overall structure having desired characteristicsin terms of electrical and thermal conductivity and the like.

[0012] To better understand the complexity of the above considerations,we present a brief description of graphite and the manner in which it istypically processed to form flexible sheet materials. Graphite, on amicroscopic scale, is made up of layer planes of hexagonal arrays ornetworks of carbon atoms. These layer planes of hexagonally arrangedcarbon atoms are substantially flat and are oriented or ordered so as tobe substantially parallel and equidistant to one another. Thesubstantially-flat, parallel, equidistant sheets or layers of carbonatoms, usually referred to as graphene layers or basal planes, arelinked or bonded together and groups thereof are arranged incrystallites. Highly-ordered graphite materials consist of crystallitesof considerable size: the crystallites being highly aligned or orientedwith respect to each other and having well ordered carbon layers. Inother words, highly ordered graphites have a high degree of preferredcrystallite orientation. It should be noted that graphites, bydefinition, possess anisotropic structures and thus exhibit or possessmany characteristics that are highly directional, e.g. thermal andelectrical conductivity and fluid diffusion. Sometimes this anisotropyis an advantage and at others it can lead to process or productlimitations.

[0013] Briefly, graphites may be characterized as laminated structuresof carbon, that is, structures consisting of superposed layers orlaminae of carbon atoms joined together by weak van der Waals forces. Inconsidering the graphite structure, two axes or directions are usuallynoted, to wit, the “c” axis or direction and the “a” axes or directions.For simplicity, the “c” axis or direction may be considered as thedirection perpendicular to the carbon layers. The “a” axes or directionsmay be considered as the directions parallel to the carbon layers or thedirections perpendicular to the “c” direction. The graphites suitablefor manufacturing flexible graphite sheets possess a very high degree oforientation.

[0014] As noted above, the bonding forces holding the parallel layers ofcarbon atoms together are only weak van der Waals forces. Naturalgraphites can be chemically treated so that the spacing between thesuperposed carbon layers or laminae can be appreciably opened up so asto provide a marked expansion in the direction perpendicular to thelayers, that is, in the “c” direction, and thus form an expanded orintumesced graphite structure in which the laminar character of thecarbon layers is substantially retained.

[0015] Graphite flake which has been chemically or thermally expandedand more particularly expanded so as to have a final thickness or “c”direction dimension which is as much as about 80 or more times theoriginal “c” direction dimension, can be formed without the use of abinder into cohesive or integrated sheets of expanded graphite, e.g.webs, papers, strips, tapes, or the like (typically referred to as“flexible graphite”). The formation of graphite particles which havebeen expanded to have a final thickness or “c” dimension which is asmuch as about 80 times or more the original “c” direction dimension intointegrated flexible sheets by compression, without the use of anybinding material, is believed to be possible due to the mechanicalinterlocking, or cohesion, which is achieved between the voluminouslyexpanded graphite particles.

[0016] In addition to flexibility, the sheet material, as noted above,has also been found to possess a high degree of anisotropy with respectto thermal and electrical conductivity and fluid diffusion, comparableto the natural graphite starting material due to orientation of theexpanded graphite particles substantially parallel to the opposed facesof the sheet resulting from very high compression, e.g. roll pressing.Sheet material thus produced has excellent flexibility, good strengthand a very high degree of orientation. There is a need for processingthat more fully takes advantage of these properties.

[0017] Briefly, the process of producing flexible, binderlessanisotropic graphite sheet material, e.g. web, paper, strip, tape, foil,mat, or the like, comprises compressing or compacting under apredetermined load and in the absence of a binder, expanded graphiteparticles which have a “c” direction dimension which is as much as about80 or more times that of the original particles so as to form asubstantially flat, flexible, integrated graphite sheet. The expandedgraphite particles that generally are worm-like or vermiform inappearance will, once compressed, maintain the compression set andalignment with the opposed major surfaces of the sheet. The density andthickness of the sheet material can be varied by controlling the degreeof compression.

[0018] Lower densities are often thought to be advantageous wheresurface detail requires embossing or molding. Lower densities aid inachieving good detail. However, strength, thermal conductivity andelectrical conductivity are generally favored by more dense sheets.Typically, the density of the sheet material will be within the range offrom about 0.04 g/cc to about 1.4 g/cc. It would be desirable to have aprocess that would enable varying densities as needed. Currenttechnology does not lend itself easily to meet these challenges.

[0019] Flexible graphite sheet material made as described abovetypically exhibits an appreciable degree of anisotropy due to thealignment of graphite particles parallel to the major opposed, parallelsurfaces of the sheet, with the degree of anisotropy increasing uponroll pressing of the sheet material to increased density. Inroll-pressed anisotropic sheet material, the thickness, i.e. thedirection perpendicular to the opposed, parallel sheet surfacescomprises the “c” direction and the directions ranging along the lengthand width, i.e. along or parallel to the opposed, major surfacescomprises the “a” directions and the thermal, electrical and fluiddiffusion properties of the sheet are very different, by orders ofmagnitude typically, for the “c” and “a” directions.

[0020] This considerable difference in properties, i.e. anisotropy, isdirectionally dependent, and is in need of control for optimizingproperties for many applications. In applications such as electrodes forfuel cells, it would be of advantage if the electrical resistancetransverse to the major surfaces of the flexible graphite sheet (“c”direction) were decreased, and this might be achieved by utilizingsheets of higher density. However, high density sheets might inhibiteffective embossing. And, the embossing (or other shaping) operationcould cause further, undesirable variations in properties. It would bedesirable to enable a process which provided for both.

[0021] Thermal properties, e.g., thermal conductivity, of a flexiblegraphite sheet in a direction parallel to the major surfaces of theflexible graphite sheet is relatively high, while it is relatively lowin the “c” direction, transverse to the major surfaces. Again, it wouldbe desirable to alter this property in a manner consistent witheffective embossing.

[0022] Flexible graphite sheet can be provided with channels, which arepreferably smooth-sided, and which pass between the parallel, opposedsurfaces of the flexible graphite sheet and are separated by walls ofcompressed expanded graphite. When the flexible graphite sheet functionsas an electrode in an electrochemical fuel cell, it is placed so as toabut the ion exchange membrane so that the “tops” of the walls of theflexible graphite sheet abut the ion exchange membrane. Thus, there areproducts requiring dense flat surfaces and embossed—surfaces, productrequirements presenting different demands on a flexible graphite sheetstarting material having predictable properties that can be optimizedfor many articles of uniform construction but must be compromised inothers.

[0023] There remains a need in the art for a material which can be usedin preparing flexible graphite articles, particularly those that areembossed with particular patterns thereon and, especially, to methodsand materials enabling the preparation of shaped elements havingpredetermined density gradation and/or detail necessary for theirintended functions, thereby facilitating quality control while also,preferably improving performance per unit weight for the final articlesproduced. If available, such needed methods and materials would aid theformation of an array of final products, some of which are useful ascomponents in electrochemical supercapcitors and fuel cells.

SUMMARY OF THE INVENTION

[0024] Accordingly, it is an object of the invention to provide amaterial that can be used in preparing a flexible graphite article thatis embossed with a particular pattern thereon.

[0025] It is a more particular object of the invention to providemethods and materials enabling the preparation of shaped elements havingpredetermined characteristics, such as density gradation and/or detail.

[0026] It is another object of the invention to enable the production ofa variety of shaped component parts having variations in density and/ordetail necessary for their intended functions while facilitating qualitycontrol.

[0027] It is another object of the invention to provide a process foradvantageously utilizing the anisotropic characteristics of flexiblegraphite sheet in the manufacture of shaped objects and preformed blanksfor use in their production.

[0028] It is another specific object of the invention to providematerials and methods helpful in improving performance per unit weightfor a variety of shaped articles useful as components in electrochemicalsupercapcitors and fuel cells.

[0029] It is another object of the invention to provide materials andmethods which enable maintenance of high densities consistent with goodthermal and electrical conductivity while permitting molding or shapingthe materials with good detail.

[0030] It is yet another object of the invention to provide materialsand methods which provide a more uniform distribution of graphite weightper unit area in a substrate for preparing a shaped component and thefinal component itself.

[0031] It is a still further object of the invention to providematerials and methods which enable molding and shaping of the materialsin high detail while assuring good control of tolerances for productdensity, porosity, thickness and other characteristics.

[0032] These and other objects are accomplished by the presentinvention, which provides a material useful as a substrate for anembossed flexible graphite sheet and methods for preparing materials ofthis type.

[0033] The material of the invention is useful as a substrate forpreparing articles such as an embossed flexible graphite sheet, thematerial comprising a composite flexible graphite sheet comprising aplurality of zones, e.g., layers, of flexible graphite sheet, at leastone of which is preferably resin-impregnated, wherein: at least one ofsaid plurality of zones has a characteristic, e.g., density and/or voidcondition or the like, distinct from at least one other of saidplurality of zones. In some forms, the material will further include alayer of a diverse material, e.g., of a nonporous or foraminous materialinterposed between at least two layers of a plurality of layers. Anonporous sheet, foil or film can be employed for a useful purpose suchas sealing or the like where it does not interfere with the end use. Aforaminous material can be selected for any of a variety of addedadvantages and is typically one selected from the group consisting ofperforated films and foils, and woven and nonwoven fabrics and webs. Inone embodiment, at least one of said plurality of layers has a densityhigher of at least about 1.1 g/cc and at least one has a density higherof less than about 1.0 g/cc.

[0034] According to the process of the invention, composite materialsuseful for a number of purposes including as substrates for formingembossed flexible graphite articles are prepared to include a pluralityof zones of resin-impregnated flexible graphite sheet, wherein at leastone of said plurality of zones has a characteristic, e.g., density orvoid condition, distinct from at least one other of said plurality ofzones. The process preferably comprising: placing a first layer ofresin-impregnated flexible graphite sheet adjacent, e.g., on top of, atleast one other layer of resin-impregnated flexible graphite sheethaving a characteristic different from said first layer.

[0035] Many preferred and alternative aspects of the invention aredescribed below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The present invention will be better understood and itsadvantages more apparent in view of the following detailed description,especially when read with reference to the appended drawings, wherein:

[0037]FIG. 1 is a schematic of a composite material useful for a numberof purposes including as a substrates for forming embossed flexiblegraphite articles;

[0038]FIG. 2 is a schematic of a process for embossing a material asshown in FIG. 1; and

[0039]FIG. 3 is a cross-sectional view of a representative shapedarticle having one shaped surface and one flat surface as will beimproved by the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0040] The invention will be illustrated and explained in thisdescription by specific reference to the production of shaped elementssuitable for use in electrochemical devices such as fuel cells of thePEM type and double-layer capacitors wherein a variation in physicalproperties in terms of density gradation and/or shaped detail isimportant. It will be recognized, however, that while this descriptionis made for illustrative purposes, the invention has broaderapplicability and is useful in the production of materials for manyother end uses.

[0041] Central to all of the embodiments of the invention is theprovision of a material having of a plurality of zones, e.g., layers orpartial layers, of flexible graphite sheet (also termed “foil”) andproviding for diversity of characteristics between the zones. By theterm “zone” we refer to a defined volume, comprised of any vertically orhorizontally-oriented, three-dimensional space filled with a graphitesheet material of one characteristic and bounded by a void (as at a top,bottom or side) or another graphite sheet or a diverse material (e.g., anonporous or foraminous material). By the term “characteristic” we meana measurable property or characterizing feature. Among the most commonproperties measured in the use of flexible graphite foil are: electricalconductivity, electrical resistivity, thermal conductivity, density,void condition, area weight, gas permeability, water permeability,particle size, type of graphite starting material, presence andcomposition of filler materials, degree of graphite intumescence, latentintumescence potential, the presence and composition of intercalationcompounds, the presence and composition of residues, resin content andcomposition, degree of resin drying, degree of resin cure or cross-linkand the like.

[0042] Suitable flexible graphite sheet material is commerciallyavailable in a variety of grade 5 and thicknesses and densities for avariety of end uses. The preferred products will contain a suitablebinder, and in others a binder will not be employed. These materials areavailable under the trademarks GRAFOIL® and GRAFCELL® from Graftech,Inc. The sheet material, or at least one zone of it, is preferablyimpregnated with resin, such as an acrylic-, epoxy- or phenolic-basedresin system, prior to shaping, such as by layering, with or withoutcutting, and then pressing with at least one shaping member, such as byembossing. Advantageously, the resin is cured during or after the stepof shaping the flexible graphite sheet. The resin content of theresin-impregnated flexible graphite sheet material is preferably atleast about 5%, and more preferably at least about 10%, by weight. Insome cases, the material can be shaped or otherwise subjected toconsolidating pressure and then impregnated.

[0043] In the course of this description, we will refer to flexiblegraphite sheet or foil, and we mean to use these terms interchangeably.The term “flexible graphite sheet” in this context is meant to refer toan article made of compressed, exfoliated graphite either by itself orwith one or more fillers or binders, wherein parallel surfaces ofparticles of graphite are oriented principally in a plane perpendicularto the “c” direction of the graphite particles and the thickness of thearticle in the direction parallel to the “c” direction is less thanabout 1.5 mm. The invention will have particular advantage when dealingwith thin sheets, namely those of less than about 1.0 mm in thickness.Sheets having thicknesses in the range of from about 0.05 to about 0.5mm will have particular advantage for some applications. For others,thicknesses of from 0.2 to 0.75 mm will be preferred. In yet others therange can be a narrow low range of from about 0.075 to about 0.2 mm. Theflexible graphite sheet material is preferably of low area weight, e.g.,from about 0.001 to about 1.4 g/cm², to facilitate impregnation andsubsequent handling in roll form. In some cases, area weights of lessthan 0.5, e.g., from 0.1 to 0.4, will be useful. In others area weightsof from above 0.5 to 1.4, e.g., from 0.6 to 1.0, will be useful.Advantageously, thinner materials may be rolled into coils andtransported as a continuous sheet rather than cut into pieces forshipment. This facilitates processing and saves material.

[0044] Briefly, before describing the invention in detail, reference ismade to FIG. 1, which illustrates, schematically, a composite material10 of the invention, useful for a number of purposes including as asubstrate for forming embossed flexible graphite articles. The compositeis made of a plurality of layers, here two layers, 12 and 14. However,the number of layers is not limited to two and up to ten or more layerscan be employed to advantage. Indeed, an important aspect of theinvention is that it provides an opportunity to improve the productthrough the reduction of variation in area weight. When randomlyselected, the variation in a laminate will be reduced by the square rootof the number of layers in the laminate, through deliberate selection,further improvements can be achieved. Impregnated and dried materials ina condition for embossing may be assembled to accumulate any desiredtarget density. When then formed the assembled pieces will combine asone. With suitable selection in density for the layers of a laminate,the assembly will yield a more true representation of the die cavity andshow fewer discontinuities resulting from shear during forming thanwould a single layer. For example, nine layers of 8-mg/cm² may be formedinto a single 72-mg/cm² molding blank.

[0045] A material of the type shown in FIG. 1 can be formed continuouslyfrom rolled sheets of resin-impregnated flexible graphite foil or byhand lay-up. In one automated process, as illustrated in FIG. 2, twosheets, 12 and 14, are brought together with contact pressure sufficientto promote substantial interfacial sheet contact by pressing betweenrollers 20 and 22. The resulting composite 24 can be distributed in thisform as a roll or cut sheet. If desired, the composite can be pressedunder sufficient pressure and heat, with or without the addition ofadditional binder or a solvent for a binder already present. The Figureshows the composite sheet material being directly fed to a shapingstation wherein the sheet is supported against backing roller 30 whereit is pressed by embossing roller 32 to provide a material as shown, forexample, in FIG. 3. This Figure shows, as a cross-sectional view of arepresentative shaped article 40 having one shaped surface 42 and oneflat surface 44. According to the invention, the proper selection ofmaterials for the composite have enabled the density of the top surface42 to be shaped with high conformance to the detail on the embossingroll 32, while the opposite surface 44 is smooth and dense.

[0046] For some applications, it can prove advantageous to use thelowest density layers of the laminate where the embossing detail is mostcritical and very high density where the surface is to be continuous.Other reasons for density variation also occur. Appropriate inter layersof suitable materials, such as nonreactive sheet material, such as anonporous sheet, foil or film can be employed for a useful purpose suchas sealing or the like where it does not interfere with the end use andin some cases will be perforated or expanded, such as an insert of meshfabric can provide improved toughness, or conductivity and thus addvalue to the laminate.

[0047] Before describing the manner in which the invention improvescurrent materials, a brief description of graphite and its formationinto flexible sheets, which will become the component parts of thematerials of the invention is in order.

[0048] Preparation of Flexible Graphite Foil

[0049] Graphite is a crystalline form of carbon comprising atomscovalently bonded in flat layered planes with weaker bonds between theplanes. By treating particles of graphite, such as natural graphiteflake, with an intercalant of, e.g. a solution of sulfuric and nitricacid, the crystal structure of the graphite reacts to form a compound ofgraphite and the intercalant. The treated particles of graphite arehereafter referred to as “particles of intercalated graphite.” Uponexposure to high temperature, the intercalant within the graphitedecomposes and volatilizes, causing the particles of intercalatedgraphite to expand in dimension as much as about 80 or more times itsoriginal volume in an accordion-like fashion in the “c” direction, i.e.in the direction perpendicular to the crystalline planes of thegraphite. The exfoliated graphite particles are vermiform in appearance,and are therefore commonly referred to as worms. The worms may becompressed together into flexible sheets that, unlike the originalgraphite flakes, can be formed and cut into various shapes and providedwith small transverse openings by deforming mechanical impact.

[0050] Graphite starting materials suitable for use in the presentinvention include highly graphitic carbonaceous materials capable ofintercalating organic and inorganic acids as well as halogens and thenexpanding when exposed to heat. These highly graphitic carbonaceousmaterials most preferably have a degree of graphitization of about 1.0.As used in this disclosure, the term “degree of graphitization” refersto the value g according to the formula:$g = \frac{3.45 - {d(002)}}{0.095}$

[0051] where d(002) is the spacing between the graphitic layers of thecarbons in the crystal structure measured in Angstrom units. The spacingd between graphite layers is measured by standard X-ray diffractiontechniques. The positions of diffraction peaks corresponding to the(002), (004) and (006) Miller Indices are measured, and standardleast-squares techniques are employed to derive spacing which minimizesthe total error for all of these peaks. Examples of highly graphiticcarbonaceous materials include natural graphites from various sources,as well as other carbonaceous materials such as carbons prepared bychemical vapor deposition and the like. Natural graphite is mostpreferred.

[0052] The graphite starting materials used in the present invention maycontain non-carbon components so long as the crystal structure of thestarting materials maintains the required degree of graphitization andthey are capable of exfoliation. Generally, any carbon-containingmaterial, the crystal structure of which possesses the required degreeof graphitization and which can be intercalated and exfoliated, issuitable for use with the present invention. Such graphite preferablyhas an ash content of less than 20% (weight), and for electrochemicaluses less than 6% is often desired. More preferably, the graphiteemployed for the present invention will have a purity of at least about94%. In the most preferred embodiment, the graphite employed will have apurity of at least about 99% for electrochemical fuel cell uses.

[0053] A common method for manufacturing graphite sheet is described byShane et al in U.S. Pat. No. 3,404,061, the disclosure of which isincorporated herein by reference. In the typical practice of the Shaneet al. method, natural graphite flakes are intercalated by dispersingthe flakes in a solution containing e.g., a mixture of nitric andsulfuric acid, advantageously at a level of about 20 to about 300 partsby weight of intercalant solution per 100 parts by weight of graphiteflakes (pph). The intercalation solution contains oxidizing and otherintercalating agents known in the art. Examples include those containingoxidizing agents and oxidizing mixtures, such as solutions containingnitric acid, potassium chlorate, chromic acid, potassium permanganate,potassium chromate, potassium dichromate, perchloric acid, and the like,or mixtures, such as for example, concentrated nitric acid and chlorate,chromic acid and phosphoric acid, sulfuric acid and nitric acid, ormixtures of a strong organic acid, e.g. trifluoroacetic acid, and astrong oxidizing agent soluble in the organic acid. Alternatively, anelectric potential can be used to bring about oxidation of the graphite.Chemical species that can be introduced into the graphite crystal usingelectrolytic oxidation include sulfuric acid as well as other acids.

[0054] In a preferred embodiment, the intercalating agent is a solutionof a mixture of sulfuric acid, or sulfuric acid and phosphoric acid, andan oxidizing agent, i.e. nitric acid, perchloric acid, chromic acid,potassium permanganate, hydrogen peroxide, iodic or periodic acids, orthe like. The intercalation solution may also contain metal halides suchas ferric chloride, and ferric chloride mixed with sulfuric acid, or ahalide, such as bromine, as a solution of bromine and sulfuric acid orbromine, in an organic solvent.

[0055] The quantity of intercalation solution may range from about 20 toabout 150 pph and more typically about 50 to about 120 pph. After theflakes are intercalated, any excess solution is drained from the flakesand the flakes are water-washed. Alternatively, the quantity of theintercalation solution may be limited to between about 10 and about 50pph, which permits the washing step to be eliminated as taught anddescribed in U.S. Pat. No. 4,895,713, the disclosure of which is alsoherein incorporated by reference.

[0056] The particles of graphite flake treated with intercalationsolution can optionally be contacted, e.g. by blending, with a reducingorganic agent selected from alcohols, sugars, aldehydes and esters whichare reactive with the surface film of oxidizing intercalating solutionat temperatures in the range of 25° C. and 125° C. Suitable specificorganic agents include hexadecanol, octadecanol, 1-octanol, 2-octanol,decylalcohol, 1,10-decanediol, decylaldehyde, 1-propanol,1,3-propanediol, ethyleneglycol, polypropylene glycol, dextrose,fructose, lactose, sucrose, potato starch, ethylene glycol monostearate,diethylene glycol dibenzoate, propylene glycol monostearate, glycerolmonostearate, dimethyl oxylate, diethyl oxylate, methyl formate, ethylformate, ascorbic acid and lignin-derived compounds, such as sodiumlignosulfate. The amount of organic reducing agent is suitably fromabout 0.5 to 4% by weight of the particles of graphite flake.

[0057] The use of an expansion aid applied prior to, during orimmediately after intercalation can also provide improvements. Amongthese improvements can be reduced exfoliation temperature and increasedexpanded volume (also referred to as “worm volume”). An expansion aid inthis context will advantageously be an organic material sufficientlysoluble in the intercalation solution to achieve an improvement inexpansion. More narrowly, organic materials of this type that containcarbon, hydrogen and oxygen, preferably exclusively, may be employed.Carboxylic acids have been found especially effective. A suitablecarboxylic acid useful as the expansion aid can be selected fromaromatic, aliphatic or cycloaliphatic, straight chain or branched chain,saturated and unsaturated monocarboxylic acids, dicarboxylic acids andpolycarboxylic acids which have at least 1 carbon atom, and preferablyup to about 15 carbon atoms, which is soluble in the intercalationsolution in amounts effective to provide a measurable improvement of oneor more aspects of exfoliation. Suitable organic solvents can beemployed to improve solubility of an organic expansion aid in theintercalation solution.

[0058] Representative examples of saturated aliphatic carboxylic acidsare acids such as those of the formula H(CH₂)_(n)COOH wherein n is anumber of from 0 to about 5, including formic, acetic, propionic,butyric, pentanoic, hexanoic, and the like. In place of the carboxylicacids, the anhydrides or reactive carboxylic acid derivatives such asalkyl esters can also be employed. Representative of alkyl esters aremethyl formate and ethyl formate. Sulfuric acid, nitric acid and otherknown aqueous intercalants have the ability to decompose formic acid,ultimately to water and carbon dioxide. Because of this, formic acid andother sensitive expansion aids are advantageously contacted with thegraphite flake prior to immersion of the flake in aqueous intercalant.Representative of dicarboxylic acids are aliphatic dicarboxylic acidshaving 2-12 carbon atoms, in particular oxalic acid, fumaric acid,malonic acid, maleic acid, succinic acid, glutaric acid, adipic acid,1,5-pentanedicarboxylic acid, 1,6-hexanedicarboxylic acid,1,10-decanedicarboxylic acid, cyclohexane-1,4-dicarboxylic acid andaromatic dicarboxylic acids such as phthalic acid or terephthalic acid.Representative of alkyl esters are dimethyl oxylate and diethyl oxylate.Representative of cycloaliphatic acids is cyclohexane carboxylic acidand of aromatic carboxylic acids are benzoic acid, naphthoic acid,anthranilic acid, p-aminobenzoic acid, salicylic acid, o-, m- andp-tolyl acids, methoxy and ethoxybenzoic acids, acetoacetamidobenzoicacids and, acetamidobenzoic acids, phenylacetic acid and naphthoicacids. Representative of hydroxy aromatic acids are hydroxybenzoic acid,3-hydroxy-1-naphthoic acid, 3-hydroxy-2-naphthoic acid,4-hydroxy-2-naphthoic acid, 5-hydroxy-1-naphthoic acid, 5- hydroxy-2-naphthoic acid, 6- hydroxy-2- naphthoic acid and 7- hydroxy-2-naphthoicacid. Prominent among the polycarboxylic acids is citric acid.

[0059] The intercalation solution will be aqueous and will preferablycontain an amount of expansion aid of from about 1 to 10%, the amountbeing effective to enhance exfoliation. In the embodiment wherein theexpansion aid is contacted with the graphite flake prior to or afterimmersing in the aqueous intercalation solution, the expansion aid canbe admixed with the graphite by suitable means, such as a V-blender,typically in an amount of from about 0.2% to about 10% by weight of thegraphite flake.

[0060] After intercalating the graphite flake, and following theblending of the intercalant coated intercalated graphite flake with theorganic reducing agent, the blend is exposed to temperatures in therange of 25° to 125° C. to promote reaction of the reducing agent andintercalant coating. The heating period is up to about 20 hours, withshorter heating periods, e.g., at least about 10 minutes, for highertemperatures in the above-noted range. Times of one half hour or less,e.g., on the order of 10 to 25 minutes, can be employed at the highertemperatures.

[0061] The thus treated particles of graphite are sometimes referred toas “particles of intercalated graphite.” Upon exposure to hightemperature, e.g. temperatures of at least about 160° C. and especiallyabout 700° C. to 1000° C. and higher, the particles of intercalatedgraphite expand as much as about 80 to 1000 or more times their originalvolume in an accordion-like fashion in the c-direction, i.e. in thedirection perpendicular to the crystalline planes of the constituentgraphite particles. The expanded, i.e. exfoliated, graphite particlesare vermiform in appearance, and are therefore commonly referred to asworms. The worms may be compressed together into flexible sheets that,unlike the original graphite flakes, can be formed and cut into variousshapes and provided with small transverse openings by deformingmechanical impact as hereinafter described.

[0062] Flexible graphite sheet and foil are coherent, with good handlingstrength, and are suitably compressed, e.g. by roll-pressing, to athickness of about 0.05 mm to 4 mm and a typical density of about 0.1 to1.4 grams per cubic centimeter (g/cc). From about 1.5-30% by weight ofceramic additives can be blended with the intercalated graphite flakesas described in U.S. Pat. No. 5,902,762 (which is incorporated herein byreference) to provide enhanced resin impregnation in the final flexiblegraphite product. The additives include ceramic fiber particles having alength of about 0.15 to 1.5 millimeters. The width of the particles issuitably from about 0.04 to 0.004 mm. The ceramic fiber particles arenon-reactive and non-adhering to graphite and are stable at temperaturesup to about 1100° C., preferably about 1400° C. or higher. Suitableceramic fiber particles are formed of macerated quartz glass fibers,carbon and graphite fibers, zirconia, boron nitride, silicon carbide andmagnesia fibers, naturally occurring mineral fibers such as calciummetasilicate fibers, calcium aluminum silicate fibers, aluminum oxidefibers and the like.

[0063] Preparation of Resin-Impregnated Flexible Graphite Foil Theinvention is facilitated by the use of resin-impregnated flexiblegraphite foils in at least one zone. To prepare it by impregnatingbefore final consolidation into the material of the invention, flexiblegraphite sheet is treated with resin and if needed dried afterabsorption of the resin. This is a useful binder to ensure cohesivenessof the final product and, after consolidating or curing, enhances thefluid resistance and impermeability, improves the and handling strength,i.e. stiffness, of the flexible graphite sheet, as well as “fixing” themorphology of the sheet. Suitable resin content is preferably at leastabout 5% by weight, more preferably about 10 to 35% by weight, andsuitably up to about 60% by weight. Resins found especially useful inthe practice of the present invention include acrylic-, epoxy- andphenolic-based resin systems, or mixtures thereof. Suitable epoxy resinsystems include those based on diglycidyl ether of bisphenol A (DGEBA)and other multifunctional resin systems; phenolic resins that can beemployed include resole and novolac phenolics. Typically, but notnecessarily, the resin system is solvated to facilitate application intothe flexible graphite sheet.

[0064] In a typical resin impregnation step, the flexible graphite sheetis passed through a vessel and impregnated with the resin system from,e.g. spray nozzles, the resin system advantageously being “pulledthrough the mat” by means of a vacuum chamber. The resin is thereafterpreferably dried, reducing the tack of the resin and theresin-impregnated sheet, which has a starting density of about 0.1 toabout 1.1 g/cc, can thereafter processed to change the void condition ofthe sheet.

[0065] One form of apparatus for continuously forming resin-impregnatedand calendered flexible graphite sheet is shown in InternationalPublication No. WO 00/64808, the disclosure of which is incorporatedherein by reference.

[0066] It is an advantage of the invention that layers or other zones,e.g., partial layers such as precut inserts, can have void conditionsdiffering in adjacent zones. By void condition is meant the percentageof the sheet represented by voids, which are generally found in the formof entrapped air. Generally, this is accomplished by the application ofpressure to the sheet (which also has the effect of densifying thesheet) so as to reduce the level of voids in the sheet, for instance ina calender mill or platen press. Advantageously, the flexible graphitesheet is densified to a density of at least about 1.3 g/cc (although thepresence of resin in the system can be used to reduce the voids withoutrequiring densification to so high a level).

[0067] Preparation of Composite Graphite Materials

[0068] The invention provides a material useful as a substrate for anembossed flexible graphite sheet and comprises a composite flexiblegraphite sheet comprising a plurality of zones (e.g., layers or partiallayers) of resin-impregnated, flexible graphite sheet. At least one ofthe plurality of zones has a characteristic different than at least oneother of said plurality of zones. Among the various characteristics thatare advantageously varied between zones according to the invention areall of those above or as may otherwise be measured and can be related toproperties of a final shaped article or an effect on the process formaking it. Typically, the invention can provide useful properties in thecharacteristics of final shaped articles when the difference in thecharacteristics referred to is greater than about 5%, and more typicallyfrom about 10% up to about 200%, based on the smaller of the two values.In many cases, differences of from about 20 to about 75% on this basisare valuable. In others difference values up to as much as 500% can havea particular utility.

[0069] In one form, the composite material of the invention will beprovided with a unique combination of properties by further including alayer of a diverse material interposed between at least two layers ofsaid plurality of layers. The diverse material can be a foraminousmaterial, e.g., selected for any of a variety of added advantages and istypically one selected from the group consisting of perforated films andfoils, and woven and nonwoven fabrics and webs of plastic, metal and/ornatural composition. In all cases, the materials are employed to providea unique ability to control final shaped article characteristics, e.g.,density and surface properties, by preselecting layers of differentcharacteristic, e.g., density or void condition, to comprise the layersof the composite. In other cases, the diverse material can be anonporous film, sheet or foil of plastic or metal to aid in sealing andfor other useful properties. By properly utilizing suitable inter layerswith impregnated materials, an external binder can be eliminated,further decreasing electrical and thermal resistance.

[0070] Referring again to FIG. 1, a composite material 10 of theinvention, is shown schematically, and is of type useful for a number ofpurposes including as a substrates for forming embossed flexiblegraphite articles. The composite is made of a plurality of layers, heretwo layers, 12 and 14. However, the number of layers is not limited totwo, and up to ten or more layers can be employed to advantage. Indeed,an important aspect of the invention is the opportunity to improve theproduct through the reduction of variation in area weight. When randomlyselected the variation in a laminate will be reduced by the square rootof the number of layers in the laminate, through deliberate selection,further improvements can be achieved. Impregnated and dried materials ina condition for embossing may be assembled to accumulate any desiredtarget density. When then formed the assembled pieces will combine asone. With suitable selection in density for the layers of a laminate,the assembly will yield a more true representation of the die cavity andshow fewer discontinuities resulting from shear during forming thanwould a single layer. For example, nine layers of 8-mg/cm² may be formedinto a single 72-mg/cm² molding blank.

[0071] A material of the type shown in FIG. 1 can be formed continuouslyfrom rolled sheets of resin-impregnated flexible graphite foil or byhand lay-up. In one automated process, as illustrated in FIG. 2, twosheets, 12 and 14 are brought together with contact pressure sufficientto promote substantial interfacial sheet contact by pressing betweenrollers 20 and 22. The resulting composite 24 can be distributed in thisform as a roll or cut sheet. If desired, the composite 24 can be pressedunder sufficient pressure and heat, with or without the addition ofadditional binder or a solvent for the binder already present.

[0072] In some cases, it will prove advantageous to use the lowestdensity layers of the laminate where the embossing detail is mostcritical and very high density where the surface is to be continuous.Appropriate inter layers of suitable materials, such as nonreactivesheet material, preferably perforated or expanded, or insert mesh fabriccan provide improved toughness, or conductivity and thus add value tothe laminate. Such materials, preferably foraminous in structure, canadd useful properties to the final product and the material of theinvention as well. These materials can add strength and/or enhance anyof the above-noted characteristics for desired end uses. A nonporoussheet, foil or film can be employed for a useful purpose such as sealingor the like where it does not interfere with the end use.

[0073] The void condition, as with a variety of other characteristics ofa flexible graphite sheet can be used advantageously to control and/oradjust the morphology and functional characteristics of one or morezones in a composite material of the invention, and, thus, the finalembossed article. For instance, thermal and electrical conductivity,permeation rate and leaching characteristics can be affected andpotentially controlled by controlling the void condition (and, usually,the density) of one or more zones extending in any desired direction,prior to embossing. Thus, if a set of desired characteristics of thefinal embossed article is recognized prior to manipulation of the voidcondition, the void condition can be tailored to achieve thosecharacteristics, to the extent possible. By selecting a sheet ofpredetermined void condition as made in this manner or from a variety ofsheets sorted by void volume or other characteristic after forming andtesting, a composite material of the invention can be formed bylaying-up, or cutting or otherwise shaping and then laying up, materialsof different void condition or other characteristic such as densities,and thereby achieve zones of predetermined characteristics.

[0074] Most advantageously, especially when the final embossed articleis intended for use as a component in an electrochemical fuel cell, azone in a composite material requiring optimization of thermal and/orelectrical conductivity, or the like, the resin-impregnated flexiblegraphite sheet can be manipulated so as to be relatively void-free.Generally, this can be accomplished by achieving a density of at leastabout 1.4 g/cc, more preferably at least about 1.7 g/cc, indicating arelatively void-free condition, which leads to production of an embossedarticle having a relatively high anisotropy ratio (potentially on theorder of about 150 and higher).

[0075] In other cases, a composite material may be formed to facilitateflow during shaping to assure proper physical shape, the void conditionwill be selected to permit flow to the extent necessary with the finalshaped material having a final void condition consistent with theelectrical and thermal properties required. Where a low anisotropy ratiois desired, such as in certain thermal interface applications, a highervoid condition density is preferred, which generally corresponds to adensity in the range of about 1.1 to about 1.3 g/cc (again, depending onthe presence/level of resin in the system).

[0076] The invention, rather than embossing a single sheet of graphitefoil to achieve the desired properties, forms a composite material. FIG.2 shows a composite sheet material 24 being directly fed to a shapingstation wherein the sheet is supported against backing roller 30 whereit is pressed by embossing roller 32 to provide a material 40 as shown,for example, in schematic cross section in FIG. 3. This Figure shows, asa cross-sectional view of a representative shaped article having oneshaped surface 42 and one flat surface 44. According to the invention,the proper selection of materials for the composite have enabled thedensity of the top surface 42 to be shaped with high conformance to thedetail on the embossing roll 32, while the opposite surface 44 is smoothand dense. In other arrangements, the materials forming the individualzones can be laid up in a stationary compression mold and pressed bymoving the mold parts together to the desired extent in terms ofpressure and/or separation.

[0077] The composite material of the invention as illustratedschematically in cross section in FIG. 1, is comprised of zones ofdifferent characteristics, being passed through an embossing apparatus.In the case of FIG. 1, the top layer 12 has a density suitable for someembossing applications, e.g., between 0.1 and 1.4 grams/cc, while thebottom layer 14 is relatively dense, e.g., from 1.4 to 1.8 grams/cc.Depending on the nature of the resin system employed, and especially thesolvent type and level employed (which is advantageously tailored to thespecific resin system, as would be familiar to the skilled artisan), avaporization drying step may be included prior to the embossing step. Inthis drying step, the resin impregnated flexible graphite sheet isexposed to heat to vaporize and thereby remove some or all of thesolvent, without effecting cure of the resin system. In this way,blistering during the curing step, which can be caused by vaporizationof solvent trapped within the sheet by the densification of the sheetduring surface shaping, is avoided. The degree and time of heating willvary with the nature and amount of solvent, and is preferably at atemperature of at least about 65° C. and more preferably from about 80°C. to about 95° C. for about 3 to about 20 minutes for this purpose.

[0078] The composite material can be used to create optimizedmorphological characteristics. By utilizing layers with different bindercontents, relative flow levels can be changed, leading to preferentialflow of certain layers to fill surface features on any embossingsurface. The invention also provides a material useful in creatingembossed materials without material sticking to the forming surfaces. Byusing formable surface layers with limited/zero resin content, surfacedefinition can be obtained without adherence of the material to theembossing elements. This includes both platen press and continuousforming operations. This eliminates external and internal mold releaseagents, for example silicone release sprays, which may affectperformance in the end-use environment.

[0079] An intermediate layer may comprise one or more layers of aforaminous material of any suitable construction and material. Typicallyit will be one selected from the group consisting of woven and nonwovenfabrics or webs, having any suitable structure of individual fibers orthreads, films or foils, batts, knitted fabric, and the like, of anysuitable metal, plastic or fiber of organic or inorganic composition.Nonwoven fabrics or webs can be formed from many processes such as forexample, melt blowing processes, spin bonding processes, and bondedcarded web processes. By definition, the foraminous material will permitthe passage of a fluid from one surface through to the other. Thematerial will have openings through it to facilitate this, but the size,shape and arrangement of the openings can be selected as desired for thefinal article characteristics. It is an advantage of the invention thatforaminous materials of all kinds can be employed. Desirable foraminousmaterials will have integral means or shapes that facilitate mechanicalor other physical bonding between the graphite layers and the foraminouslayer or layers.

[0080] The resulting embossed graphite sheet can be used in a variety ofapplications, including as a component in an electrochemical fuel cell.

[0081] The following examples are presented to further illustrate andexplain the invention and are not intended to be limiting in any regard.Unless otherwise indicated, all parts and percentages are by weight.

EXAMPLE 1

[0082] This example illustrates the production of a composite stockmaterial, suitable for embossing to form fuel cell fluid flow fieldplates, and the like. Two sheets of resin impregnated graphite foil areprovided. One consists of a material that has a thickness of 1.0 mm anda density of 0.75 grams/cc. The other has a thickness of 1.0 mm and adensity of 1.0 grams/cc. The sheets are layered and then compressedbetween a pair of nip rolls spaced at 1.5 mm, with the result that thetwo sheets are consolidated into a single composite having zones ofdiffering densities through thickness.

EXAMPLE 2

[0083] The procedure of Example 1 is repeated, but this time the firstlayer is 0.25 mm in thickness, has a density of 1.0 grams/cc and is notresin impregnated.

EXAMPLE 3

[0084] The procedure of Example 1 is repeated, but this time one of thelayers is not a solid sheet but rather contains cut-out regions, in thisone case, a frame of material. This allows for the creation of a singlecomposite having zones of differing densities through thickness andin-plane.

EXAMPLE 4

[0085] The procedure of Example 1 is repeated, but this time a thirdlayer, a copper 16-mesh woven copper screen is placed between the sheetof graphite foil and the nip of the rollers is set at a gap of 1.5 mm.

EXAMPLE 5

[0086] The procedure of Example 1 is repeated, but this time thefirst-mentioned of the sheets of graphite foil is replaced by animpregnated one including a_ceramic fiber_additive (see U.S. Pat. No.5,902,762). The fiber containing layer provides for further manipulationof the anisotropy of the layers, allowing for greater designflexibility.

EXAMPLE 6

[0087] The procedure of Example 1 is repeated, but this time two layersof 0.25 mm copper foil are used to clad the resin impregnated graphitelayers.

EXAMPLE 7

[0088] The procedure of Example 1 is repeated, but this time nine layersof 8-mg/cm² may be formed into a single 72-mg/cm² molding blank. Whenrandomly selected, the variation in a laminate will be reduced by thesquare root of the number of layers in the laminate. They are subjectedsufficiently to consolidate them sufficiently for handling prior toembossing, which will combine as one.

[0089] The above description is intended to enable the person skilled inthe art to practice the invention. It is not intended to detail all ofthe possible variations and modifications that will become apparent tothe skilled worker upon reading the description. It is intended,however, that all such modifications and variations be included withinthe scope of the invention that is defined by the following claims. Theclaims are intended to cover the indicated elements and steps in anyarrangement or sequence that is effective to meet the objectivesintended for the invention, unless the context specifically indicatesthe contrary.

What is claimed is:
 1. A material useful as a substrate for an embossedflexible graphite sheet, the material comprising a composite flexiblegraphite sheet comprising a plurality of zones of flexible graphitesheet, wherein: at least one of said plurality of zones has acharacteristic different from at least one other of said plurality ofzones.
 2. A material according to claim 1 wherein at least one of saidzones comprises resin-impregnated graphite sheet.
 3. The material ofclaim 1 wherein the property is one selected from the group consistingof: electrical conductivity, thermal conductivity, density, voidcondition, area weight, gas permeability, water permeability, particlesize, type of graphite starting material, presence and composition offiller materials, degree of graphite intumescence, latent intumescencepotential, the presence and composition of intercalation compounds, thepresence and composition of residues, resin content and composition,degree of resin drying, degree of resin cure or cross-link, and thelike.
 4. The material of claim 1 wherein the thickness of the at leastone of said plurality of zones is less than about 0.1 mm.
 5. Thematerial of claim 1 wherein zones are comprised of sheets less thanabout 2.0 mm in thickness.
 6. The material of claim 1 wherein thedifference in the characteristics is greater than about 5%.
 7. Thematerial of claim 1 comprising as at least one of said zones, a graphitesheet material 2 of area weight of from about 0.001 to about 2.0 g/cm².8. The material of claim 1 further including a layer of a diversematerial interposed between at least two layers of said plurality oflayers.
 9. The material of claim 8 wherein the diverse material is aforaminous one selected from the group consisting of woven and nonwovenfabrics or webs or metal foils.
 10. The material of claim 8 wherein thediverse material is a nonporous sheet, film or foil.
 11. The material ofclaim 1 wherein at least one of said plurality of zones has a density ofat least about 1.1 g/cc and at least one of said plurality of layers hasa density of less than about 1.0 g/cc.
 12. The material of claim 1wherein the resin is present at a level of at least about 5% in theflexible graphite sheet.
 13. The material of claim 12 wherein the resincomprises an acrylic-based resin system, an epoxy-based resin system ora phenolic-based resin system.
 14. A process for preparing a compositematerial useful as a substrate for forming an embossed flexible graphitesheet, comprising a plurality of zones of flexible graphite sheet,wherein at least one of said plurality of zones has a density higherthan at least one other of said plurality of regions, comprising;placing a first portion of resin-impregnated flexible graphite sheet incontact with at least one other portion of resin-impregnated flexiblegraphite sheet having a characteristic different from said first portionto form a composite; and subjecting the composite to consolidatingpressure.
 15. A process according to claim 14 wherein at least one ofsaid zones comprises resin-impregnated graphite sheet.
 16. A processaccording to claim 14 wherein the property is one selected from thegroup consisting of: electrical conductivity, thermal conductivity,density, void condition, area weight, gas permeability, waterpermeability, particle size, type of graphite starting material,presence and composition of filler materials, degree of graphiteintumescence, latent intumescence potential, the presence andcomposition of intercalation compounds, the presence and composition ofresidues, resin content and composition, degree of resin drying, degreeof resin cure or cross-link, and the like.
 17. A process according toclaim 14 wherein the thickness of the at least one of said plurality ofzones is less than about 0.1 mm.
 18. A process according to claim 14wherein zones are comprised of sheets less than about 2.0 mm inthickness.
 19. A process according to claim 14 wherein the difference inthe characteristics is greater than about 5%.
 20. A process according toclaim 14 wherein at least one of said zones, comprises a graphite sheetmaterial of area weight of from about 0.001 to about 2.0 g/cm².
 22. Aprocess according to claim 14 further including a layer of a diversematerial interposed between at least two layers of said plurality oflayers.
 23. A process according to claim 14 wherein the diverse materialis a foraminous one selected from the group consisting of woven andnonwoven fabrics or webs or metal foils.
 24. A process according toclaim 23 wherein the diverse material is a nonporous sheet, film orfoil.
 25. A process according to claim 14 wherein at least one of saidplurality of zones has a density of at least about 1.1 g/cc and at leastone of said plurality of layers has a density of less than about 1.0g/cc.
 26. A process according to claim 14 wherein the resin is presentat a level of at least about 5% in the flexible graphite sheet.
 27. Aprocess according to claim 14 wherein the resin comprises anacrylic-based resin system, an epoxy-based resin system or aphenolic-based resin system.